Why 6th October / 10th June? The Hatter, whom Alice was told was mad, was depicted in illustrations sporting a top hat which had a piece of paper sticking from with the words "In this style 10/6". The 10/6 in this instance, no doubt, referred to the cost of the hat in old English money, i.e. "ten shillings and sixpence". However, the founders of Mad Hatter Day decided that this could also represent the day on which Mad Hatter Day should be held. So, it is therefore held on 10/6 - October 6th or June 10th depending on how you read it. So in the US it is October 6th, and in other places such as Great Britain it is June 10th.